1. Technological Field
The present invention relates to a long-life welding electrode and a fixing structure for the same, a welding head, and to a welding method.
2. Background Technology
Conventionally, types of electrodes for welding are divided to those each having a sharp tip and those each having a flat tip. And in electrodes for welding having a sharp tip, the form remarkably changes and the durability is low, while, in those each having a flat chip, the form little changes, but the arc discharge characteristic is remarkably degraded, and for the reasons as described above, in both types of electrode for welding, a frequency of exchange is high and the work efficiency is low.
In a conventional type of electrode 901 having a flat tip as shown in FIG. 9(b), there are a number of points at which a distance between the welding electrode 901 and a welded object 910 becomes shortest (arc grounding point:), and a point, from which arc discharge is generated (arc discharge point: ∘), is generated at various points, so that an arc used for welding can not be kept stable.
Also, a welding electrode is worn out along a form of an equipotential surface, and it has been found out that a welding electrode having a sharp tip (with an angle from 30 to 60 degrees), is easily worn out and it is impossible to executed welding for a long time under stable conditions.
On the other hand, it has been tried to develop a technology of adding around 2 weight % of ThO2 (thoria) in a mother material for a welding electrode (W) for the purpose to improve durability of the electrode.
When thoria is added, sometimes the durability may be improved, but the durability is not always improved. That is to say, the effect provided by adding thoria is not constant.
Conventionally, a welding electrode is fixed with a screw. That is to say, as shown in FIG. 2, a welding electrode 201 is fixed to a fixing base 202 by inserting the welding electrode 201 into an inserting section 204 of a fixing base 202 having the inserting section 204 and also passing a setting screw 203 through a screw hole provided in a side of the fixing base 202.
With the conventional type of fixing structure as described above, however, a welding electrode is degraded.
Generally argon is used as a welding gas, and argon is poor in its thermal conductivity, and high current value is required for welding, so that a temperature of the welding electrode increases and durability of the electrode is worsened.
Further, a welding gas is supplied via a welding power supply unit with a resin-based material emitting a large quantity of gas used in piping in the power supply unit, and also a resin-based material is used for a tube for connection among a gas supply system, a welding power supply unit, and a welding head, so that atmosphere is worsened during welding and the welding electrode may be oxidized and degraded (Refer to FIG. 6).
Engineers are required for replacement of a degraded electrode, and many engineers are required for the conventional type of electrodes each frequently requiring replacement, and also a long time is required for this work for replacement, so that the work efficiency is low, which does now allow welding with high reliability.
It is an object of the present invention to provide an electrode for welding and a fixing structure for the same, a welding head, and a welding method which make it possible to improve durability of a welding electrode, because of forms and materials thereof as well as of atmosphere for welding, to largely reduce a frequency of exchange of welding electrodes and also reduce a number of engineers and a time required for exchanging welding electrodes, to improve the work efficiency, and to enable wending with high reliability for a long time.
(1) A welding electrode according to the present invention is characterized in that a tip section thereof has a curved surface. More specifically, this curved surface should preferably be like a form of equipotential surface which is vertical against an electric line of force generated between a welding electrode and a material to be welded.
With the configuration as described above, as shown in FIG. 9(a), an arc discharge position is kept constant, and wearing of the electrode can be suppressed by uniformly generating a current generated from a welding electrode, which makes it possible to improve durability of the welding electrode.
It is preferable from the view point of further improving durability of a welding electrode that the curved surface is an arch-formed one with a diameter from 0.05 mm or more to 0.3 mm or less.
(2) The welding electrode according to the present invention is characterized in that at least one type of oxide selected from a group comprising lanthana, yttrium, and ceria is added to a mother material for the electrode.
The present inventors made strenuous efforts to understand how a life of a welding electrode is decided. As a result, it was found out that a life of wiring (xcfx84) used in the semiconductor industry is applicable to a life of a welding electrode, and the equation for a life of wiring (xcfx84) is as follows:
xcfx84=(E0/(xcfx81J2))exp(Ea/kT)
wherein J indicates a current density, xcfx81 indicates are sistivity of wiring, E0 indicates a constant specified to wiring, K indicates a Boltzmann constant, T indicates a temperature of wiring, and Ea indicates energy for activation.
Herein, assuming that a material for the welding electrode is constant, a resistivity xcfx81 of the welding electrode, E0, Ea, and k are kept unchanged, and also assuming that the temperature T on the welding electrode when a type of welding gas and a welding current are changed with a distance between the welding electrode and an object to be welded and a melting area of the object to be welded kept unchanged is constant, a life (xcfx84) of the welding electrode is expressed by the following equation:
xcfx84=(1/J2)A
wherein A=(E0/xcfx84) exp (Ea/kT), and from this expression it can be understood that the life of a welding electrode is inverse proportion to a square of the current density. Herein, if a form of the welding electrode""s tip is as shown in FIG. 1 and is kept unchanged, it is possible to substitute the current density for a current value, and also as shown in FIG. 3 describing below, if a distance between the welding electrode and an object to be welded is fixed and a melting area of an object to be welded and a form of the welding electrode are kept unchanged, a life of the welding is decided only by a current value.
Also, a current value during welding is lowered because, when a gas with a high thermal conductivity (such as hydrogen or helium) is added, an arc polar is made thinner because of the thermal pinch effect and a density of electrons irradiated to the object to be welded becomes higher.
As described above, a life of a welding electrode is decided by a current density, and as a current density is smaller, a life of a welding electrode becomes longer.
A current density is expressed by the expression of J=AT2 exp (xe2x88x92"PHgr"/kT). Herein J indicates a current density, A indicates a thermoelectron discharge constant, T indicates a temperature of an electrode, K indicates a Boltzmann constant, and "PHgr" indicates a work function. For this reason, to improve the thermoelectron performance during arc discharge, a work with a small work function should be used. For that purpose, an oxide with a small work function may be added to a mother material for a welding electrode. However, if a melting point of the added oxide is low, the oxide is evaporated during welding, and when the welding electrode is used frequently, the electrode is degraded. So by using an oxide with a small work function and also with a high melting point and a high boiling point to improve the thermoelectron discharge performance as well as to prevent evaporation of a welding electrode due to frequency use thereof, durability of the welding electrode can be improved.
Herein a melting point, a boiling point, and a work function of each material are shown in Table 1.
As described above, conventionally it has been tried to improve the durability of a welding electrode by adding thoria. However, the correlation between addition of thoria and improvement of durability as described above has not been clarified. Also as shown in Table 1, a value of thoria""s work function extends in a board range. This can be regarded as one the reasons whey the effect obtained by adding thoria is not stable.
In the present invention, an oxide with a low work function and a high melting point is added to a mother material for a welding electrode. Generally an oxide with a work function smaller than that of tungsten and a melting point higher than 2000xc2x0 C. is used. More specifically, any of lanthana, yttria, and zirconia is added.
Addition of an oxide should preferably be in a range from 1 wt % to 5 wt %, and more preferably be in a range from 2 wt % to 5 wt %. When the addition is 1 wt % or more, durability of an electrode is improved more remarkably. When the addition is more than 5 wt %, as the melting point is lower than that of tungsten as the mother material, sometimes the electrode itself may be reduced. For this reason, the addition should preferably be in a range from 2 wt % to 5 wt %.
When an oxide is added, a life of a welding electrode has correlation with a current value during welding, and the durability can be improved by reducing a current value, so that durability of a welding electrode can be improved by adding a gas with a high thermal conductivity as a welding gas to lower a current value loaded to the electrode during welding by making use of the thermal pinch effect and also to lower a temperature of the welding electrode.
Rmax at a surface of a welding electrode should preferably be in a range from 3 xcexcm or more to 10 xcexcm or less. When a surface of a welding electrode is made smooth, it becomes possible to suppress emission of a gas from the welding electrode as well as to prevent degradation of the electrode with the durability of the electrode improved. For that purpose, Rmax at a surface of the electrode should preferably be 10 xcexcm or less. Also if the Rmax is more than 10 xcexcm, an arc is generated and the arc can not disadvantageously be kept stable, but the arc during welding can be kept stable when the value is 10 xcexcm or less. It should be noted that, even when Rmax is 3 xcexcm or less, the effect is saturated and the cost becomes disadvantageously high.
(3) The structure for fixing a welding electrode according to the present invention is characterized in that a fixed section of the welding electrode is inserted via a thermally conductive material into an inserting section of a fixing base for a welding electrode to be inserted therein and a peripheral surface of the fixed section of the welding electrode is uniformly contacted to the fixing base to fix the welding electrode to the fixing base.
As shown in FIG. 2, conventionally a welding electrode 201 is fixed to a fixing base with a screw, but in this fixing system, the welding electrode 201 is degraded as described above.
The present inventors made strenuous efforts for finding out the cause, and found out the cause that a clearance with a width of around 100 xcexcm exists between the welding electrode 201 and the fixing bench 202 and the clearance degrades the welding electrode. Namely, this clearance hinders emission of heat from the welding electrode 201, which causes degradation of the welding electrode 201.
In the present invention, as shown in FIG. 3, a contact area between a welding electrode 301 and a fixing base 302 is made larger by providing a thermally conductive material 304 between the welding electrode 301 and the fixing base 302, which makes easier emission of heat generated by welding with temperature increase in the welding electrode 301 suppressed, deformation of the electrode prevented, and also degradation of the electrode prevented. With this feature, the durability of a welding electrode can be improved.
As a thermally conductive material, for instance, such a material as Cu, Au, Ag, or Pt is used.
To provide the thermally conductive material between a welding electrode and a fixing base, for instance, a powder-state thermally conductive material solved in an organic solvent may be flowing into a section between the welding electrode and the fixing base and then the material is dried.
(4) In the structure for fixing a welding electrode according to the present invention, for instance, the fixing base is divided to several pieces, and a fixed section of a welding electrode is fixed by holding the fixed section with the divided pieces of the fixing base.
In the present invention, a fixing base is divided to several portions, and a welding electrode is fixed by being held between the divided portions of the fixing base. Because of this feature, the welding electrode and fixing base are contacted to each other without any clearance therebetween, and the performance for emitting heat from a welding electrode is improved. In other words, emission of heat generated during welding is made easier with temperature increase in the welding electrode suppressed and also deformation of the electrode prevented, thus degradation of the electrode being prevented. Because of this feature, durability of a welding electrode can be improved.
It should be noted that, also in this case, it is preferable to provide a thermally conductive material between the welding electrode and the fixing base. For the purpose to provide a thermally conductive material between the welding electrode and the fixing base, for instance, a powder-state thermally conductive material solved in an organic solvent may be applied to a section between the welding electrode and the fixing base and then dried.
(5) A welding method according to the present invention is characterized in that welding is executed by using any of (1) a mixed gas of argon and helium, (2) a mixed gas of helium and hydrogen, and (3) a mixed gas of argon, helium, and hydrogen is used as a welding gas.
Herein a content of helium in each mixed gas should preferably be in a range from 1 to 90%.
In the present invention, a welding current is reduced with durability of a welding electrode improved by adding hydrogen or helium each having a high thermal conductivity or a mixture of hydrogen and helium. Also as hydrogen is a reduction gas, oxidization of a welding electrode is prevented, thus degradation of the welding electrode being prevented.
A content of helium should preferably be in a range from 1 to 90%, more preferably in a range from 1 to 20%, and further preferably in a range from 0.5 to 10%.
Tungsten in a welding electrode is easily oxidized, and the oxidized tungsten degrades the arc discharge performance.
The durability of a welding electrode can be improved by using an all-metallic (stainless) gas supply system in place of a resin-based one discharging much gas for a supply tube for a welding gas so that no impurities (such as moisture) will not be included in the welding gas for the purpose to prevent the electrode from being degraded. Especially, as stainless steel with an impassive layer made of a chrome oxide formed on a top surface thereof emits a gas quite little, it is preferable to use the stainless steel.
As described above, with the present invention, the durability of a welding electrode is improved with the work efficiency in welding improved, and also a time doe replacement required in the conventional technology and a number of required operators can be reduced and welding can be executed for a long time with high reliability.